The present invention relates to a process for making an array of tapered photopolymerized waveguides.
Waveguides, also known in the art as light transmissive devices or lightguides, are used in display devices, such as for example projection display devices, off screen display devices, and direct view displays. See for example U.S. Pat. No. 3,218,924 and U.S. Pat. No. 3,279,314 to Miller and U.S. Pat. No. 4,767,186 to Bradley, Jr. et al. Such displays are used in a wide range of applications including computer terminals, airplane cockpit displays, automotive instrument panels, televisions, and other devices that provide text, graphics, or video information.
Processes for making waveguides for use in display devices are known. For example, U.S. Pat. Nos. 3,218,924; 3,279,314; and U.S. Pat. No. 4,767,186 teach that projection screens may be manufactured by known processes and list compression molding, rejection molding, extrusion, hot roller pressing, casting, and photopolymerization. U.S. Pat. No. 3,767,445 teaches a method for producing integrated optical waveguides by making a die having a surface matching the desired uniform cross section waveguide shape, embossing a transparent plastic substrate with the die, and coating the embossed substrate with a high refractive index material.
U.S. Pat. No. 5,230,990 teaches a process for making an optical waveguide array of light transmissive cores with uniform cross section throughout the core length. A resist master is formed first by exposing a photoresist layer through a photomask, dissolving the exposed areas, and heating the non-exposed areas to form parallel rows of semicircular shaped protuberances. The resist is heated until it is melted and the liquid surface tension draws it into a line of a semicircular cross section or a hemispherical bead. Thus, semicircular grooves and hemispherical dots are the only shapes attainable with this method. A stamper is then formed from the resist master by sputtering an electroconductive film on the resist master to form a metal master and then forming a metallic film on the metal master to form a stamper having parallel rows of semicircular shaped protuberances. The stamper is then used to form the optical waveguide array by exposing a low refractive index material on the stamper, removing the cured layer, putting a high refractive index material into the semicircular shaped grooves of the cured layer, curing the high refractive index, putting a low refractive material on the high refractive index material, and curing. See also U.S. Pat. Nos. 4,902,086; 5,113,471; and 5,136,678.
Kokai Patent Publication 245106 published Oct. 31, 1991 teaches a process of making an optical plate of a substrate having high refractive index waveguides with uniform cross section throughout the waveguide length, wherein the waveguides are separated by low refractive index material. The optical plate is made by placing high refractive index photosensitive resin on a substrate, coveting the photosensitive resin with a low oxygen permeable sheet, and exposing the photosensitive resin to ultraviolet light through a photomask and the low oxygen permeable sheet to form uniform cross section waveguides. The unexposed resin and low oxygen permeable sheet are removed and low refractive index material is placed around the waveguides.
The 245106 process is disadvantageous because it produces waveguides which when used on a display, would suffer from low contrast and changes in visual chromaticity as the viewing angle changes. In order to obtain good collection of light at the input end of the waveguides, they must be close packed at the input end, as taught in Kokai 245106. Since the cross-section is uniform, this close packed arrangement persists throughout the optical plate, along the length of the waveguides. This results in difficulty in removing the unexposed material from the wall area during the development step and additionally, does not provide space on the output end of the plate for contrast improving materials to be added. Moreover, waveguides with uniform cross section do not increase the angular distribution of the light which passes through them. It is often highly desirable that the waveguides should cause such an increase in angular distribution as is taught in commonly assigned U.S. patent application Ser. No. 86,414 filed Jul. 1, 1993.
Other processes for making optical waveguides having uniform cross section are disclosed in U.S. Pat. Nos. 4,783,136 and 5,138,687; European Patent Publication 357396 published Mar. 7, 1990; Kokai Patent Publication 24121 published Feb. 2, 1993; and Kokai Patent Publication 119203 published May 18, 1993.
U.S. Pat. No. 4,712,854 teaches two processes for forming optical waveguides suitable for connection to optical fibers. The first process involves a first ultraviolet light exposure step to form a refractive index profile in the depthwise direction of the photopolymerization material and a second ultraviolet light exposure step to form a refractive index profile in the widthwise direction to form an optical waveguide having good matching characteristics relative to a graded index type optical fiber. This process is disadvantageous because two ultraviolet light exposure steps are required. In addition, no relief image is formed and the waveguide consists of a higher refractive index region within a sheet of continuous polymer film, or a lamination of several of such films. Optical waveguide propagation is therefore strictly limited to the plane of the film, never to propagation normal to the film surface.
Kokai Patent Publication 42241 published Sep. 19, 1986 teaches a process of making a lightguide array by moving a substrate through a container of monomer solution while exposing the solution to ultraviolet light through a photomask so that uniform cross section lightguide bodies form on the substrate. During the process, the monomer solution being exposed is positioned between the photomask and substrate and the photomask is positioned between the ultraviolet light and the monomer solution. During the exposure, the substrate is moved perpendicularly away from the light source. The substrate is then removed from the container and a lower refractive index material is poured around the rod-shaped lightguide bodies and cured. Tapered lightguide bodies are formed by positioning a lens between the photomask and monomer solution and moving the lens, thereby altering the mask image magnification, so that the lightguide bodies taper in a direction away from the substrate toward the lens, photomask, and ultraviolet light. The complicated moving mask and lens system makes this process disadvantageous to use. Also, this process is incapable of making tapered lightguide bodies wherein the center-to-center distance between light input surfaces of adjacent bodies is substantially equal to the center-to-center distance between light output surfaces thereof. As a result, the area of the input and output surfaces of the array cannot be equal. The array area at the tapered end is decreased by the square of the reduction ratio of the individual elements, as is clear from the diagrams of Kokai 42241. This method is therefore of no use for creating tapered waveguide arrays for use in display applications such as liquid crystal displays (LCD's), as it is highly undesirable to shrink the size of the display. Further, the cross sections of individual elements in an array created by a moving lens are not uniform. Those which are close to the optical axis of the lens system have a different cross-sectional profile than those lying at the perimeter of the array. This effect is also clearly shown in the diagrams of Kokai 42241. As a result, the individual elements of the array will have nonuniform optical properties, which is highly undesirable in display systems where such nonuniformities would degrade the image qualities.
As such, the need exists in the art for a simple process for making an array of tapered photopolymerized waveguides wherein the array of tapered photopolymerized waveguides has improved properties.